Frequency-modulated signal distortion reducing circuit



Aug. 22, 1950 J. HAANTJES 2,519,455

FREQUENCY-MODULATED SIGNAL DISTORTION REDUCING CIRCUIT Filed April 27,1946 INVENTOR X 'KGENT Patented Aug. 22, 1950 2,519,455

UNITED STATES PATENT orncs FREQUENCY-MODULATED SIGNAL DISTORTIONREDUCING CIR/QUIT Johan Haantjes, Eindhoven, Netherlands, ass gnor, bymesne assignments, to Hartford National Bank and Trust Company,Hartford, Conn., as trustee Application April 27, 1946, Serial No.665,394 In the Netherlands November :13, 1942 Section 1, Public Law 690,August 8, 1946 Patent expires November 13, 1962 1 Claim. (=01. 178 44) 12 In the transmission of frequency-modulated wherein w is the circuitfrequency. Let wo be the oscillations through a circuit including one orcentral frequency and: more tuned circuits, distortion occurs due to thebehaviour of the resonant circuit. This distortion L g g =l (2) resultsfrom the production of spurious frequencies and more particularly thirdharmonics of the and assume that: frequencies present in the usefulspectrum. This ,0

distortion may attain an undue value if the fre- 8 (3) quency deviationdoes not materially exceed the I highest frequency to be transmittedthen inserting Equations 2 and 3 in Equation 1:

The invention provides a simple means of elimij L 2L nating thisdistortion both in such cases and in imm other cases where spuriousfrequencies may be The phase Shift p of a complex impedance generateddue to circuit distortion. 7

According to the invention, a series circuit is Z a+b is given by therelation coupled in parallel with the resonant circuit or =am tan 2 acircuits responsible for the production of spurious frequencies. Thisseries circuit comprises a damping resistance and a series combinationof 3 2: f 2:23;: 12 3 the Impedance Z in Equa' an inductance and acapacitance tuned to the frequency of the resonant circuit. Theinductance I: are tan h -1 1 1 B3 In of the series combination, thedamping reo i 'zR wo L1C2 R sistance R. and the capacitance Ci of theresonant circuit are so proportioned as to satisfy the re- 5=JL2 (3)lationship: w e

L2=0.32R2C1 B: e wo--Aw Quin-A60 6 w Aw co (c -m ms The accompanyingdrawing represents by way Assuming. of example a resonant circuit whichforms part of a system (not represented) for transmitting, 3O a Awreceiving or amplifying frequency-modulated os- Then;

cillations, which circuit is made as free from distortion as possible inthe manner according to il (7) the invention.

The circuit comprises a coil L1 with a parallel I t n Equation 7 inEquation 5: condenser C1 by means of which the circuit is C 1 tuned tothe central frequency We of the system. l tan f crg Aw-l-smmo (8) Thetransmission band of the circuit is widened 2 1 2 by means of a parallelresistance R.

In series with this damping resistance B. there is connected, accordingto the invention, a cir- Developing w in the power expansion of Am thecoeflicient of A403 has the following value:

cuit which is tuned to the central frequency W0 8 l 8 C R L 8 and whichconsists of the inductance L2 and a LC'QRE m) =0 (9) condenser C2 inseries. The relationship of the elements L2-C1 and R should be such thatwhich (308113016111; must be zero.

It follows: L2=0.32R C1 L +C R L2 0 R 0 (10( This equation is derived asfollows:

For the impedance Z of the circuit shown in 59 Rearrangmg Equatin2 thedrawing, it is easily shown that: 1

1 020! 1 1 ,Tw h i 1 1 j C', -T z Inserting Equations 11 31d 12 inEquation 10:

LPr-BGQRLr-CJR U Solving Equation is:

L2: '0.32R2C1 14 If the relation of Equation 14 is sa.-t istied-, thenthe third harmonic produced by circuit'distortion is reduced to anegligible amount, This result is achieved even under the condition-61aparatively small frequency deviation.

What I claim is:

A frequency modulated wave transmission circuit arrangement comprising aiaariilleI memos: cillatory circuit having an inductive element? fi-fitla capacitive element tuned to the central fre quency of the wave to betransmitted, a series tuned oscillatory circuit having an inductiveelement and a capacitive element tuned to the fie quency of the wave tob'etransmitted, a resistive element; a'series connection of said seriestuned oscillatory circuit and said resistance, aparallel connection ofsaid series connected series tuned com iz'iiiERENcEs CITED foiiownigreferences are of record in the file or this" 'fiatent:

UNITED STATES PATENTS Name Date v Riiiia Mar. 26, I940 Rente 2,195,695

